Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. The source of the acoustic energy can be generated by explosions, air guns vibrators, and the like. The energy source is positioned on or near the surface of the earth. Each time the energy source is activated it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be detected at many locations on the surface as a function of travel time. The sensors used to detect the returning seismic energy commonly include geophones, accelerometers, and hydrophones. The returning seismic energy is recorded as a continuous signal representing displacement, velocity, acceleration, or other recorded variation as a function of time. Multiple combinations of energy source and sensor can be subsequently combined to create a near continuous image of the subsurface that lies beneath the survey area. One or more sets of seismic signals may be assembled in the final seismic survey.
Technology continues to increase resolution and complexity of seismic systems such as high fidelity vibroseis seismic acquisition including ZENSEIS™. Vibroseis is a method used to propagate energy signals into the earth over an extended period of time as opposed to the near instantaneous energy provided by impulsive sources. The data recorded through vibroseis must be correlated to convert the extended source signal into an impulse. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or shaker unit mounted on a mobile base unit, but electro-mechanical versions have also been developed. Signals transmitted through the earth are reflected and analyzed to identify changes in signal. The exact distance the vibrations travel before being reflected are unknown and the transmission rates of the vibrations through different features is unknown, thus the time from transmission of the signal to recordation of the seismic signal is the only direct measure of distance. The exact time is additionally required to extract phase data when more than one vibroseis or other vibrational source is operated simultaneously.
Global Positioning Systems (GPS) are currently used by military and civilians to accurately determine location, direction and rate of movement, as well as time. GPS systems have been used by seismic operators to accurately place source and sensors during seismic surveys and to provide an accurate time for a GPS survey as a single source of time. Other methods are then used to synchronize time between a central recorder, source, and receivers. These methods include high-precision microsecond time recorders, accurate radio-pulse transponders and receivers, as well as other methods of high accuracy time synchronization. Radio-pulse synchronization requires radio communication with a large number of source and autonomous sensors, requires a powered receiver at each sensor, and a very accurate clock or GPS timing device to obtain microsecond precision among all of the integrated devices required for seismic surveying.
Bassett, U.S. Pat. No. 3,972,019, describes accurate timing at distant points where each unit produces synchronized time pulses at time intervals which are the same in the different units and methods for synchronizing the units by direct connection with each other.
Siems and Porter, U.S. Pat. No. 4,281,403, describe a decentralized seismic data recording system including a central station and a plurality of remote seismic recording units. A master clock is provided in the central station. A local clock is provided in each remote recording unit. At the beginning of a work period, the local clocks are synchronized with the master clock. Thereafter, a plurality of seismic data recordings is made. At the end of a work period, the time difference due to tuning drift between the master clock and each respective local clock is ascertained and is recorded. The time difference is linearly prorated over the recordings made during the work period, thereby synchronizing the time base of each seismic data recording with the master clock.
Kostelnicek and Montes, U.S. Pat. No. 4,879,696, describe a method for initiating seismic data storage in an Isolated Distributed Recording System, identifying correlation between a “correlation” signal and a refracted seismic signal containing the correlation signal. Triggering the isolated distributed recorder to store incoming seismic data when the correlation signal is identified. Time Synchronization Systems may be employed that include an accurate clock or other similar timing device at the central station, and each isolated distributed recorder would also have an equally accurate timing device or “local” clock in relationship with the timing device at the central station. Usually, the local clocks are synchronized with the clock at the central station prior to the initiation of active seismic exploration activities.
Reed, et al., U.S. Pat. No. 4,885,724, describe a cableless seismic digital recording system which records seismic-trace data generated by any type of seismic source, including high energy impulsive seismic sources and low energy surface seismic sources such as vibrators. A seismometer is connected to a remotely deployed radio-controlled portable recorder which contains circuitry for sampling, digitizing, processing, storing, and recording seismic-trace data. Coded radio signals instruct each recorder to commence an operation or sequence of operations from a predetermined set of programmed instructions stored in program read only memory included in each recorder. Such operations include seismic-trace data acquisition; optional weighting and vertical stacking; normalization; recording; and seismic source initiation (abstract and p. 8, 1.25-33).
Norris, U.S. Pat. No. 6,002,339, synchronizes seismic event data using a programmable subsystem with an independent timer that can be zeroed, time transmitted with a set of instructions, and the timer can be reset when the instructions are complete. The timer is activated in response to the event and is reset following completion of the event.
Harmon, U.S. Pat. No. 6,002,640, describes use of a Series of nearly Identical Seismic Shots (SISS) to generate a system synchronization signal and instruct remote units. The timing and information contained in the SISS can be used to synchronize and communicate with the data acquisition unit(s).
Iseli, et al., U.S. Pat. No. 7,012,853, describes a method of seismic data acquisition, comprising: a) sensing acoustic energy with a plurality of sensors, each sensor providing an output indicative of the sensed energy; b) collecting a plurality of time samples of each sensor output; c) forming one or more data packets with the collected plurality of time samples: d) adding one or more characterizing bits to the data packets, the characterizing bits representing the time of only the first time sample within the data packet; e) storing the data packets in predetermined memory locations in a field unit; and f) transmitting the data packets. A synchronizing signal is included in the data packets.
Longaker, WO0116622, describes a method and apparatus for controlling vibroseis sources in survey operations. A wireless local area network establishing a communications link among vibroseis sources operating in a group may enable the group to operate independent of a remote control unit and may also provide a distributed system solution that mitigates communication difficulties between the sources and the remote control unit.
Current systems require contact, either radio or direct contact, before, during, and after the seismic survey to synchronize various parts of the system and accurately decode the information encoded in the seismic data. The systems described above use a radio pulse, pre- and post-survey synchronization, a high precision microsecond clock, or other method to synchronize the various independent recorders with the source and central recorder. The receivers, high precision time instruments, and coordinated communications described require expensive and energy intensive equipment. This is not always convenient or possible if multiple seismic measurements are required, when one or more sources or receivers are inaccessible, or when the system is dispersed over a large area.
The industry is plagued with system failures, bulky wiring systems, and lost data due to failures in timing and communication. Current systems do not provide an inexpensive and accurate method to synchronize multiple independent systems used for seismic surveys and record sufficient data to obtain a high resolution image of the geological structures. What is required are inexpensive and simple methods to synchronize equipment for seismic surveys.